An antenna array is a collection of antenna elements. Typically, the antenna elements are placed equidistant along a line. This type of antenna array is also known as a Uniform Linear Array (ULA). A common mathematical description of an ULA is:
                                          H            ⁡                          (                              λ                ,                d                ,                θ                ,                ϕ                            )                                =                                    g              ⁡                              (                                  λ                  ,                  θ                  ,                  ϕ                                )                                      ⁢                                          ∑                                  k                  =                  0                                K                            ⁢                                                w                  k                                ⁢                                  ⅇ                                                                                    2                        ⁢                        jπ                        ⁢                                                                                                  ⁢                        dk                                            λ                                        ⁢                                          cos                      ⁡                                              (                        θ                        )                                                                                                                                ,                            (        1        )            where the function g(λ,θ,φ) is an antenna element factor, d is the distance between the antenna elements, k is the element number and λ is the used wave length. The sum is often called the array factor. The antenna response of the antenna array is the array factor weighted by the element response and is typically evaluated in a plane, e.g. by sweeping the θ angle. The antenna gain response in this plane can be altered via weights wk, and a main beam can be placed in a desired direction. However, the distance between antenna elements plays an important role. The antenna response is a periodic function in e.g. θ, and the period is determined by the element distance d and the wavelength λ. An element spacing of λ/2 maps one period on the θ interval [0,π].
To avoid coupling between adjacently arranged antenna elements, the distance between elements can be increased. The drawback with an increased distance between the elements is that several periods of the gain function are mapped onto the θ interval [0,π]. This makes the antenna ambiguous in direction finding since replicas of the main beam are in the visible interval. A remedy can be to use polyphase antenna as described in the published international patent application WO 2008/066436 [reference 1], which is assigned to the present applicant and briefly described in connection with FIG. 2.
The drawback of using the polyphase antenna is cost of time, i.e. a symbol needs to be applied to the antenna ports for a complete switching cycle of the switch. This implies that the communication ratio is reduced compared to a traditional antenna configuration, in which every antenna element is fed by a signal via a dedicated radio chain as discussed in connection with FIG. 1.